This invention relates to a method for making a magnetic recording medium wherein a magnetic layer is overlaid on one surface (the front surface) of a substrate, and a backing layer is overlaid on the other surface (the back surface) of the substrate. The backing layer is formed by the application of a coating composition, which contains inorganic grains and a binder, to the back surface of the substrate.
In recent years, magnetic recording media, such as audio or video magnetic recording tapes or magnetic recording tapes for computers, have often been provided with backing layers, which improves the durability of the magnetic recording media. In general, a coating composition, which contains inorganic grains and a binder, is applied to the back surface of the substrate in order to form a backing layer on the back surface of the substrate of a magnetic recording medium. The backing layer is prepared with a series of processes wherein inorganic grains and a binder are kneaded together, the kneaded mixture thus obtained is then subjected to a dilution kneading process, and the diluted mixture thus obtained is then subjected to a dispersion process.
With conventional methods for making a magnetic recording medium, in the course of preparing a coating composition used to form a backing layer, kneading machines, such as pressure type kneading machines or roll mills, are used for the kneading process and the dilution kneading process. However, with such kneading machines, it is not always possible to knead inorganic grains and a binder under a high shearing force. Also, it is difficult to dilute the kneaded mixture, which has been obtained from the kneading process, under a high shearing force. Therefore, a uniformly diluted mixture cannot be obtained. Accordingly, when the diluted mixture is thereafter subjected to a dispersion process, it will not disperse sufficiently. In cases where the coating composition thus obtained is applied to the back surface of a substrate, the backing layer which forms will have a rough surface. When a magnetic recording medium provided with such a backing layer is wound up into a roll, rough bits of the surface of the backing layer from the back surface of one loop of the roll are transferred to the magnetic layer of the next loop of the magnetic recording medium. Also, when sheets of the magnetic recording medium are stacked one upon another, rough bits of the surface of the backing layer of one sheet of the magnetic recording medium are transferred to the magnetic layer of the next sheet of the magnetic recording medium. As a result, the surface smoothness of the magnetic layer of the magnetic recording medium is affected adversely, and therefore the electromagnetic transducing characteristics of the magnetic recording medium become poor.
As disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-41274, it has been proposed to use a two-shaft continuous kneading and mixing machine in a kneading process and a dilution kneading process for preparing a magnetic coating composition, which is used to form a magnetic layer of a magnetic recording medium. The two-shaft continuous kneading and mixing machine may also be utilized in a kneading process and a dilution kneading process for preparing a coating composition, which is used to form the backing layer of a magnetic recording medium. When a two-shaft continuous kneading and mixing machine is used, the kneading process and the dilution kneading process can be achieved under a high shearing force. Therefore, a backing layer an be obtained which has a smooth surface and in which the inorganic grains have a high packing density.
However, inorganic grains, such as grains of carbon black, have a markedly higher porosity and a larger specific surface area with respect to grain diameters than magnetic grains, which are contained in a magnetic coating composition. Therefore, in order that a uniformly diluted mixture can be obtained with a two-shaft continuous kneading and mixing machine for preparing a coating composition, which is used to form the backing layer of a magnetic recording medium, it is necessary for the ratio of the amount of the inorganic grains to the amount of the binder during the kneading process to be different from the ratio employed during the preparation of a magnetic coating composition. Also, inorganic grains, such as grains of carbon black, have a well-grown grape-like chain structure, and therefore will not disperse easily. In order for the inorganic grains to disperse easily, only a mixture which has been diluted uniformly should be subjected to the dispersion process. Therefore, it is very important that the ratio of the amount of the inorganic grains to the amount of the binder have an appropriate value. However, it has not heretofore been clarified what the ratio of the amount of the inorganic grains to the amount of the binder should be.